SPARK-2363. Clean MLlib's sample data files

(Just made a PR for this, mengxr was the reporter of:)

MLlib has sample data under serveral folders:
1) data/mllib
2) data/
3) mllib/data/*
Per previous discussion with Matei Zaharia, we want to put them under `data/mllib` and clean outdated files.

Author: Sean Owen <sowen@cloudera.com>

Closes #1394 from srowen/SPARK-2363 and squashes the following commits:

54313dd [Sean Owen] Move ML example data from /mllib/data/ and /data/ into /data/mllib/